This invention relates to high resolution scintillation counters and, more particularly, to techniques for optimizing the energy resolution of plastic scintillation counters capable of detecting gamma rays
Scintillation counters are widely used in industry, scientific research, and radiation monitoring. Scintillation counters are useful, for example, in exploring for petroleum and radioactive materials, as well as in the detection of explosive materials, many of which emit gamma rays when activated by neutrons.
The characteristic feature of a scintillation counter is the emission of light flashes by a scintillator contacting certain types of particles and radiation. Charged particles and radiation moving through a scintillator leave a trail of excited atoms which emit characteristic flashes of light. These light flashes are detected by a photosensitive device, usually a photomultiplier tube (also known as a multiplier phototube or PMT). When the sizes of the light flashes are measured in the photomultiplier, the results are commonly recorded in a multichannel pulse-height analyzer (MCA) from which one can determine the energy spectra of the particles and radiation.
One face of a scintillator is commonly placed in optical contact with the photosensitive surface of the PMT. As well known in the art, it is particularly preferred that the light coupling between these surfaces be of as high an order as practicable. See, e.g., J.B. Birks, The Theory and Practice of Scintillation Counting, Chapter 5 (1964). Reflecting material is often placed at the radial surface of the scintillator to direct as much of each light flash as possible to the photosensitive surface Scintillation counters known in the art also commonly employ an optical grease or some other specialized medium between the PMT and the scintillator in order to minimize the reflection back into the scintillator body of light rays traveling to the PMT. Unfortunately, however, such coupling means are quite sensitive to vibration and slight bending forces; thus, great care must typically be exercised in using scintillation counters. The scintillator in a scintillation counter usually comprises transparent crystalline materials, liquids, or plastics In order to function as an efficient detector, the scintillator must be transparent to its own luminescent radiation; since plastic scintillators are often meters in length, such transparency must be of high order.
Scintillators are generally fabricated from inorganic or organic materials. Inorganic scintillators are characterized by the presence of heavy elements Probably the most useful inorganic scintillator is sodium iodide activated with a small amount of thallium salt (NaI(Tl)), which is particularly useful for detecting gamma rays.
Common organic scintillators include naphthalene, anthracene, trans-stilbene, polyvinyltoluene, and a variety of other plastics. Plastic scintillators made from polyvinyltoluene have properties that make them particularly desirable in certain applications. Their fast response and relatively low cost give them significant advantages over more common inorganic detectors For example, by the simple expedient of increasing the length of a polyvinyltoluene detector one can obtain gamma ray detection efficiencies comparable to smaller but considerably more expensive NaI(Tl) detectors
Unfortunately, however, increasing the length of these plastic scintillators can also reduce their energy resolution due to the nonuniformity of light energy received at the PMT from different points within a long scintillator. Consider the scintillator 30 of FIG. 1 having a full-length reflecting material 40 on its radial surface 37 and medium 50 resulting in good optical coupling to a PMT tube 10. Two gamma rays--each having energy E--interact with the rod one at point 91 in the scintillator and the other at point 92 Those of skill in the art will appreciate that the associated Compton electron ranges are assumed to be sufficiently short that the light can be considered to originate from points 91 and 92. From this figure it can be seen that the light reaching the end of the scintillator from point 91 will be less than that from point 92 due to the greater energy losses attendant in traveling the added distance from point 91.
This difference can, of course, be minimized by employing low-absorption high-quality scintillation material with an excellent surface finish and a good outer reflecting surface. However, as recognized in the art, some difference will still be maintained between impulses reaching the PMT and energy resolution will suffer. Consequently, it would be of great advantage if some means were available for reducing the intensity difference of detected light signals produced, for example, by gamma rays of the same energy, thus improving energy resolution.